Use of conductor compositions in electronic circuits

ABSTRACT

Use of a composition comprising finely divided particles of (a) an electrically-conductive material; (b) one or more inorganic binders; and (c) aluminium, wherein components (a), (b) and (c) are dispersed in a liquid vehicle, in the manufacture of an electrically-conductive pattern on a substrate for the purpose of increasing the resistivity of said electrically-conductive pattern.

FIELD OF THE INVENTION

[0001] The present invention relates to conductor compositions with analuminium present in the composition and the use of the composition inthe manufacture of electrical components. These compositions are ofparticular use in the manufacture of demisting elements in heatedwindows, for example in automotive glazing, particularly automotivebacklights.

BACKGROUND OF THE INVENTION

[0002] The use of thick-film conductors as components in hybridmicroelectronic circuits is well known in the electronics field.Compositions for the manufacture of such components usually take theform of a paste-like solid-liquid dispersion, where the solid phasecomprises finely divided particles of a noble metal or a noble metalalloy or mixtures thereof and an inorganic binder. The liquid vehiclefor the dispersion is typically an organic liquid medium, but may alsobe an aqueous-based liquid medium. Additional materials may be added insmall quantities (generally less than about 3% by weight of thecomposition) to modify the properties of the composition and theseinclude staining agents, rheology modifiers, adhesion enhancers andsintering modifiers.

[0003] The metals used in the preparation of thick-film conductorcompositions are typically selected from silver, gold, platinium andpalladium. The metal can be used either in isolation or as a mixturewhich forms an alloy upon firing. Common metal mixtures includeplatinum/gold, palladium/silver, platinum/silver,platinum/palladium/gold and platinum/palladium/silver. The most commonsystems used in the manufacture of heating elements are silver andsilver/palladium. The inorganic binder is typically a glass orglass-forming material, such as a lead silicate, and functions as abinder both within the composition and between the composition andsubstrate onto which the composition is coated. Due to environmentalconsiderations, the use of lead-containing binders is becoming lesscommon and lead-free binders such as zinc or bismuth borosilicates arenow often employed. The role of the organic medium is to disperse theparticulate components and to facilitate the transfer of the compositiononto the substrate.

[0004] The consistency and rheology of the composition is adjusted tothe particular method of application which may comprise screen printing,brushing, dipping, extrusion, spraying and the like. Typically, screenprinting is used to apply the composition. The pastes are usuallyapplied to an inert substrate, such as an alumina, glass, ceramic,enamel, enamel-coated glass or metal substrate, to form a patternedlayer. The thick-film conductor layer is normally dried and then fired,usually at temperatures between about 600 and 900° C., to volatilise orburn off the liquid vehicle and sinter or melt the inorganic binder andthe metal components. Direct wet-firing, i.e. wherein the thick filmlayer is not dried before firing, has also been used to generate thepatterned layer.

[0005] It is, of course, necessary to connect the conductive pattern tothe other components of the electronic circuit, such as the powersource, resistor and capacitor networks, resistors, trim potentiometers,chip resistors and chip carriers. This is generally achieved by usingmetal clips, typically comprising copper, which are soldered eitherdirectly adjacent to or on top of the conductive layer. Where the clipsare soldered on top of the conductive layer, attachment is eitherdirectly onto the conductive pattern itself or onto a solderablecomposition which is overprinted onto the pattern (an “over-print”). Anover-print is generally applied only in the region of the conductivepattern to which the metal clips are attached by solder, which region isgenerally referred to as the “clip area”. The ability to solder onto theelectrically-conductive layer is an important parameter in themanufacture of heating elements since it removes the requirement for anover-print. However, the inorganic binder, which is important forbinding the paste onto the substrate, can interfere with solder wettingand result in poor adhesion of the soldered metal clips to theconductive layer. The requirements of high substrate adhesion and highsolderability (or adhesion of the metal clips to the conductive pattern)are often difficult to meet simultaneously. U.S. Pat. No. 5,518,663provides one solution to this problem by incorporating into thecomposition a crystalline material from the feldspar family.

[0006] An important application of patterned electrically-conductivelayers is in the automobile industry, and particularly in themanufacture of windows which can be defrosted and/or demisted by anelectrically-conductive grid permanently attached to the window andcapable of producing heat when powered by a voltage source. In order forthe window to defrost quickly, the circuit must be capable of supplyinglarge amounts of power from a low voltage power source, typically 12volts. For such power sources the resistivity requirement of theconductive pattern is generally in the range of from about 2 to about 5μ106 cm (5 mΩ/□ at 10 =82 m after firing). This requirement is readilymet by conductors containing noble metals, particularly silver which isthe most commonly-used material for this application.

[0007] In certain applications, a conductive composition having a higherresistivity is required. In particular, it is anticipated that theresistance requirements of window-heating elements in automobiles willshortly need to change since the automotive industry is expected toadopt the use of a 42 and 48 volt power supply in the near future. As aresult, the conductive composition used to manufacture thewindow-heating elements will be required to exhibit higher values ofresistivity, typically greater than about 10 μΩ cm, preferably greaterthan about 12 μΩ cm, particularly in the range from about 20 to about 70μΩ cm.

[0008] A number of different materials may be added to adjust thespecific resistivity of a conductive composition. For example, metalresinates such as rhodium and manganese resinates have been used toincrease resistivity, as disclosed in U.S. Pat. No. 5,162,062 and U.S.Pat. No. 5,378,408. In addition, an increase in the content of preciousmetals, particularly the platinum group metals such as platinum andpalladium, has also been used to increase the specific resistivity.Silver/palladium and silver/platinum compositions can achieveresistivity values from about 2 μΩ cm (that of a composition comprisingonly silver and binder) up to around 100 μΩ cm (for a 70:30 Pd:Agblend). Systems comprising platinum and/or palladium are, however,significantly more expensive and their use would be prohibitive inapplications requiring coverage of a large surface area, such as thewindow-heating elements used in the automotive industry. In addition, anover-print of a composition containing a high amount of silver (andtypically small amounts of filler) is generally required for certainmetal blends, such as compositions containing high palladium levels, inorder to achieve adequate solder adhesion. Conventional conductivecompositions which typically operate at resistivity values of 2 to 5 μΩcm and which are comprised predominantly of silver do not require anover-print since acceptable levels of solder adhesion can be achieved byadjusting the levels of inorganic binder.

[0009] Other, lower-cost approaches for achieving a high resistivityinvolve blending large amounts of filler into a silver-containingconductive composition to block the conductive path. Fillers aretypically inorganic materials and those commonly used are glass (whichmay be the same or different as that used for the binder) and alumina(or other metal oxides). However, such approaches tend to result in aloss of solder acceptance and solder adhesion. For example, adequatesolder adhesion can be maintained only up to a level of about 10%alumina by weight of the composition but this level is generally too lowfor an appreciable rise in resistivity. For glass-type fillers, loss ofsolder adhesion occurs at even lower levels and, again, this level istoo low for an appreciable rise in resistivity. In addition, thisproblem can not normally be ameliorated by the use of silver over-printsowing to glass migration between the layers during firing, specificallyfrom the conductive coating into the over-print.

[0010] A further advantageous property of the conductor compositions ischemical durability and resilience to exposure to varying environmentalconditions such as temperature, humidity, acid and salt. Compositionscomprising large amounts of glass filler, particularly lead-free glassfiller, are often relatively unstable to such factors.

[0011] An additional consideration is that it is desirable for theresistance of the coating composition to be substantially independent ofthe temperature of firing used in the manufacture of the patternedconductive layer. For instance, in the case of the application of aconductive composition to a glass substrate, the behaviour of thecomposition under sintering and melting should remain substantiallyconstant between the temperatures of about 620 and 680° C. Nevertheless,a change in resistance of up to about 10% between these twotemperatures, which corresponds to the behaviour of a pure silvercomposition, is generally tolerated. The use of large amounts of fillerto significantly increase resistivity results in compositions which donot generally satisfy this requirement.

[0012] A further additional consideration is that it is desirable forthe relationship between the resistivity and the amount of resistivitymodifier added to the composition to be relatively predictable and/orsubstantially linear within the target range of desired resistivities.The resistivity of compositions comprising large amounts of fillergenerally increases in an almost linear manner until a criticalconcentration is reached. At this critical concentration, theresistivity may rise very rapidly, often by an order of magnitude, whenthe level of resistivity modifier is increased by only a fraction of aweight percent. As a result, it is difficult to target specific valuesof resistivity for such compositions.

[0013] It is an object of this invention to provide higher-resistivityelectrically-conductive compositions which do not suffer from theafore-mentioned disadvantages. In particular, it is an object of thisinvention to provide an economical electrically-conductive coatingcomposition having increased resistivity while at the same timeexhibiting good solderability.

SUMMARY OF THE INVENTION

[0014] According to the invention, there is provided the use of acomposition comprising finely divided particles of (a) anelectrically-conductive material; (b) one or more inorganic binders; and(c) aluminium, wherein components (a), (b) and (c) are dispersed in aliquid vehicle, preferably an organic medium, for the purpose ofincreasing the resistivity of an electrically-conductive pattern on asubstrate.

[0015] According to a further aspect of the invention, there is providedthe use of finely-divided particles of aluminium in a compositionfurther comprising finely divided particles of (a) anelectrically-conductive material and (b) one or more inorganic bindersdispersed in a liquid vehicle, for the purpose of increasing theresistivity of an electrically-conductive pattern manufactured from saidcomposition.

[0016] According to a further aspect of the invention, there is provideda method for increasing the resistivity of an electrically-conductivepattern manufactured from a composition comprising finely-dividedparticles of (a) an electrically-conductive material and (b) one or moreinorganic binders dispersed in a liquid vehicle, said method comprisingthe incorporation of finely-divided particles of (c) aluminium into saidcomposition.

[0017] According to a further aspect of the invention there is provideda process for the manufacture of an electrically-conductive pattern,said process comprising applying to a substrate a composition comprisingfinely divided particles of (a) an electrically-conductive particles;(b) one or more inorganic binders; and (c) aluminium, said components(a), (b) and (c) being dispersed in a liquid vehicle, preferably anorganic medium, and firing the coated substrate to effect sintering ofthe finely-divided particles to the substrate. Preferably the process isa screen printing process.

[0018] According to a further aspect of the present invention there isprovided a substrate, typically a rigid substrate such as a glass(including toughened and laminated glass), enamel, enamel-coated glass,ceramic, alumina or metal substrate, having on one or more surfacesthereof an electrically-conductive pattern, said conductive patterncomprising (a) an electrically-conductive material; (b) one or moreinorganic binders; and (c) aluminium.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The compositions are suitable for use as paste compositions forforming thick-film conductive patterns on a substrate, for instance, bythe process of screen-printing. The compositions are of particular useas components in the manufacture of windows which can be defrostedand/or demisted by an electrically-conductive grid attached to thewindow, particularly for use in the automotive industry.

[0020] The composition preferably exhibits values of resistivity ofgreater than about 10 μΩ cm, preferably greater than about 12 μΩ cm,preferably in the range from about 20 to about 70 μΩ cm, and morepreferably in the range from about 20 to about 50 μΩ cm. Thus, as usedherein, the term “increasing the resistivity” means increasing theresistivity preferably to a value of resistivity of greater than about10 μΩ cm, preferably greater than about 12 μΩ cm, preferably in therange from about 20 to about 70 μΩ cm, and more preferably in the rangefrom about 20 to about 50 μΩ cm. In one embodiment, the resistivity isin the range from about 30 to about 40 μΩ cm.

[0021] As used herein, the term “finely divided” is intended to meanthat the particles are sufficiently fine to pass through a 400-meshscreen (US standard sieve scale). It is preferred that at least 50%,preferably at least 90%, and more preferably substantially all of theparticles are in the size range of 0.01 to 20 μm. Preferably, thelargest dimension of substantially all particles is no more than about10 μm and desirably no more than about 5 μm.

[0022] Preferably, the components are present in amounts such that thetotal amount of components (a), (b) and (c) is about 50 to about 95% byweight of the composition, with the liquid vehicle being present inamounts of about 5 to about 50% by weight of the composition. In apreferred embodiment, the total amount of components (a), (b) and (c) isin the range from about 60 to about 90%, preferably from about 70 toabout 85% by weight of the composition.

[0023] Compounds (a), (b) and (c) generally comprise substantially allof the solid phase material used to prepare the compositions used in theinvention.

[0024] Preferably component (a) is present in amounts of from about 30to about 99.4%, preferably from about 50 to about 98%, more preferablyfrom about 60 to about 90%, and more preferably from about 65 to about75%, by weight of the total solids present in the composition.

[0025] Preferably component (b) is present in amounts of from about 0.5to about 40%, preferably from about 1 to about 25%, preferably fromabout 2 to about 20%, preferably from about 4 to about 20% by weight ofthe total solids present in the composition. In one embodiment,component (b) is present in amounts of from about 2 to about 15%,preferably from about 4 to about 15% by weight of the total solidspresent.

[0026] Preferably component (c) is present in amounts of about 0.1 toabout 30%, preferably from about 2 to about 15%, preferably from about 7to about 15% by weight of the total solids present in the composition.In one embodiment, component (c) is present in amounts of from about 4to about 10%, and more preferably from about 5 to about 9%, by weight ofthe total solids present in the composition. In a further embodiment,component (c) is present in amounts of from about 10 to from about 15%by weight of the total solids present in the composition.

[0027] The electrically-conductive particles of component (a) can be inany form suitable for the production of the compositions used in thepresent invention. For example, electrically-conductive metallicparticles may be in the form of either metal powders or metal flakes orblends thereof. In one embodiment of the invention, the metallicparticles are a blend of powder and flake. The particle size of themetal powder or flake is not by itself narrowly critical in terms oftechnical effectiveness. However, particle size does affect thesintering characteristics of the metal in that large particles sinter ata lower rate than small particles. Blends of powders and/or flakes ofdiffering size and/or proportion can be used to tailor the sinteringcharacteristics of the conductor formulation during firing, as iswell-known in the art. The metal particles should, however, be of a sizethat is appropriate to the method of application thereof, which isusually screen printing. The metal particles should therefore generallybe no larger than about 20 μm in size and preferably less than about 10μm. The minimum particle size is normally about 0.1 μm.

[0028] The preferred metal for the electrically-conductive component (a)of the conductor composition is silver. Silver particles larger thanabout 1.0 μm impart greater colouring to the composition. It ispreferred that the compositions contain at least 50% weight silverparticles larger than 1.0 μm. The silver will ordinarily be of highpurity, typically greater than 99% pure. However, less pure materialscan be used depending on the electrical requirements of the conductivelayer or pattern. In an embodiment of the invention, component (a)comprises a mixture of silver and nickel and/or suitable derivatives. Apreferred nickel derivative suitable for use in this embodiment of theinvention is nickel boride (Ni₃B). Typically, the Ag:Ni ratio will beabout 1:1 to about 25:1, preferably at least about 1.5:1 and morepreferably about 1.5:1 to about 3:1. It will be understood by theskilled person that reference herein to the electrically-conductivecomponent (a), and to the relative amounts thereof, does not includereference to component (c) or to the relative amounts thereof, eventhough the particles of component (c) may themselves beelectrically-conductive. Equally, a reference to the particles ofcomponent (c) and the relative amounts thereof does not includereference to the electrically-conductive particles of component (a) andthe relative amounts thereof, even though the particles of component (c)may themselves be electrically-conductive.

[0029] Component (c) in the compositions used in the present inventioncomprises aluminium in one or more of the following forms:

[0030] (i) metallic aluminium particles;

[0031] (ii) particles of an aluminium-containing alloy;

[0032] (iii) a derivative of aluminium which is substantially convertedto the metal under the action of heat.

[0033] Preferably, the particles of component (c) are metallic aluminiumparticles and/or particles of an aluminium-containing alloy. Morepreferably, the particles of component (c) are metallic aluminiumparticles.

[0034] The size of the particles should generally be no larger thanabout 20 μm and preferably less than 10 μm. The minimum particle size isnormally about 0.1 μm. The particles may be spherical or spheroid orirregular in shape, in the form of a flake or a powder, or in any othersuitable morphology.

[0035] The use of component (c) as an additive provides compositionswhich exhibit (i) high resistivity; and (ii) high solder adhesion;preferably (iii) a more uniform rise in resistivity with increasingconcentration of the additive in relation to compositions in which largeamounts of filler are used to increase resistivity; and preferably also(iv) low variation of resistance with firing temperature. In addition,aluminium is a relatively inexpensive material and is an economicalmethod of increasing resistivity.

[0036] Suitable inorganic binders for use in the present invention arethose materials which upon sintering serve to bind the metal to asubstrate such as a glass (including toughened and laminated glass),enamel, enamel-coated glass, ceramic, alumina or metal substrate. Theinorganic binder, also known as a frit, comprises finely-dividedparticles and is a key component in the compositions described herein.The softening point and viscosity of the frit during firing, as well asits wetting characteristics for the metal powder/flake and thesubstrate, are important factors. The particle size of the frit is notnarrowly critical and frits useful in the present invention willtypically have an average particle size from about 0.5 to about 4.5 μm,preferably from about 1 to about 3 μm.

[0037] It is preferred that the inorganic binder is a frit having asoftening point of between about 350 and 620° C. in order that thecompositions can be fired at the desired temperatures (typically 300 to700° C., particularly 580 to 680° C.) to effect proper sintering,wetting and adhesion to the substrate, particularly a glass substrate.It is known that mixtures of high and low melting frits can be used tocontrol the sintering characteristics of the conductive particles. Inparticular, it is believed that the high temperature frit dissolves inthe lower melting frit and together they slow the sintering rate of theconductive particles as compared to pastes containing only low meltingfrit. This control of the sintering characteristics is especiallyadvantageous when the composition is printed and fired over decorativeenamels. (Decorative enamels are normally pastes comprised of one ormore pigment oxides and opacifiers and glass frit dispersed in anorganic medium.) A high melting frit is considered to be one having asoftening point above 500° C. and a low melting frit is considered to beone having a softening point below 500° C. The difference in the meltingtemperatures of the high and low melting frits should be at least 100°C. and preferably at least 150° C. Mixtures of three or more fritshaving different melting temperatures can also be used. When mixtures ofhigh and low melting frits are used in the invention, they are normallyused in ratios by weight from 4:1 to 1:4.

[0038] As used herein, the term “softening point” refers to softeningtemperatures obtained by the fibre elongation method of ASTM C338-57.

[0039] Suitable binders include lead borates, lead silicates, leadborosilicates, cadmium borate, lead cadmium borosilicates, zincborosilicates, sodium cadmium borosilicates, bismuth silicates, bismuthborosilicates, bismuth lead silicates and bismuth lead borosilicates.Typically, any glass having a high content of bismuth oxide, preferablyat least 50% and more preferably at least 70% by weight bismuth oxide,is preferred. Lead oxide as a separate phase may also be added, ifnecessary. However, due to environmental considerations, lead-freebinders are preferred. Examples of glass compositions (compositions A toI) are given in Table 1 below; the oxide components are given in weightpercent. TABLE 1 Glass Compositions A B C D E F G H I Bi₂O₃ 75.1 82.778.1 94.8 73.3 73.7 69.82 PbO 10.9 1.83 43.6 0.7 B₂O₃ 1.2 1.34 4.8 26.78.38 SiO₂ 9.3 10.3 37.5 21.7 8.6 5.2 4.7 4.8 7.11 CaO 2.4 2.68 9.7 4.00.53 BaO 0.9 ZnO 27.6 3.9 5.0 12.03 CuO 7.6 5.5 CoO 1.8 Al₂O₃ 1.1 1.224.3 5.7 2.13 Na₂O 8.7 ZrO₂ 4.0 GeO₂ 16.5 16.6

[0040] The glass binders are prepared by conventional glass-makingtechniques, by mixing the desired components (or precursors thereof,e.g., H₃BO₃ for B₂O₃) in the desired proportions and heating the mixtureto form a melt. As is well known in the art, heating is conducted to apeak temperature and for a time such that the melt becomes entirelyliquid, yet gaseous evolution has ceased. The peak temperature isgenerally in the range 1100° C.-1500° C., usually 1200° C.-1400° C. Themelt is then quenched by cooling the melt, typically by pouring onto acold belt or into cold running water. Particle size reduction can thenbe accomplished by milling as desired.

[0041] Other transition metal oxides may also be employed as part of theinorganic binder, as is well known to those skilled in the art. Oxidesor oxide precursors of zinc, cobalt, copper, nickel, manganese and ironare commonly used, particularly with substrates other than glasssubstrates, such as alumina substrates. These additives are known toimprove soldered adhesion.

[0042] The inorganic binder can also contain up to approximately 4 partsby weight basis paste of a pyrochlore-related oxide having the generalformula:

(M_(x)M′_(2-x))M″₂O_(7-Z)

[0043] wherein

[0044] M is selected from at least one of Pb, Bi, Cd, Cu, Ir, Ag, Y andrare earth metals having atomic numbers of 57-71 and mixtures thereof,

[0045] M′ is selected from Pb, Bi and mixtures thereof,

[0046] M″ is selected from Ru, Ir, Rh and mixtures thereof,

[0047] X=0-0.5, and

[0048] Z=0-1.

[0049] Pyrochlore materials have been described in detail in U.S. Pat.No. 3,583,931, the disclosure of which is incorporated herein byreference. The pyrochlore materials act as adhesion promoters for thecompositions of this invention. Copper bismuth ruthenate(Cu_(0.5)Bi_(1.5)Ru₂O_(6.75)) is preferred.

[0050] Traditionally, conductive compositions have been based on leadfrits. The elimination of lead from glass compositions to meet currenttoxicity and environmental regulations may limit the types of binderthat can be used to achieve the desired softening and flowcharacteristics, while simultaneously meeting wettability, thermalexpansion, cosmetic and performance requirements. U.S. Pat. No.5,378,406, the disclosure of which is incorporated herein by reference,describes a series of low-toxicity lead-free glasses based upon theconstituents Bi₂O₃, Al₂O₃, SiO₂, CaO, ZnO and B₂O₃, all of which may beused in the compositions described herein.

[0051] In a preferred embodiment of the invention, the frit iscomposition I in Table 1 herein.

[0052] The components (a) to (c) of the composition hereinbeforedescribed will ordinarily be dispersed into a liquid vehicle to form asemi-fluid paste which is capable of being printed in a desired circuitpattern. The liquid vehicle may be an organic medium or may beaqueous-based. Preferably the liquid vehicle is an organic medium. Anysuitably inert liquid can be used as an organic medium. The liquidvehicle should provide acceptable wettability of the solids and thesubstrate, a relatively stable dispersion of particles in the paste,good printing performance, dried film strength sufficient to withstandrough handling, and good firing properties. Various organic liquids withor without thickening agents, stabilising agents and/or other commonadditives are suitable for use in the preparation of the compositions ofthe present invention. Exemplary of the organic liquids which can beused are alcohols (including glycols); esters of such alcohols such asthe acetates, propionates and phthalates, for instance dibutylphthalate; terpenes such as pine oil, terpineol and the like; solutionsof resins such as polymethacrylates of lower alcohols; or solutions ofethyl cellulose in solvents such as pine oil and monobutyl ether ofdiethylene glycol. The vehicle can also contain volatile liquids topromote fast setting after application to the substrate.

[0053] A preferred organic medium is based on a combination of athickener consisting of ethyl cellulose in terpineol (typically in aratio of 1 to 9), optionally combined for instance with dibutylphthalate or with the monobutyl ether of diethylene glycol (sold asbutyl Carbitol™). A further preferred organic medium is based on ethylcellulose resin and a solvent mixture of alpha-, beta- andgamma-terpineols (typically 85-92% alpha-terpineol containing 8-15% betaand gamma-terpineol).

[0054] The ratio of liquid vehicle to solids in the dispersion can varyconsiderably and is determined by the final desired formulationviscosity which, in turn, is determined by the printing requirements ofthe system. Normally, in order to achieve good coverage, the dispersionswill contain about 50 to about 95%, preferably about 60 to about 90%, byweight solids, and about 5 to about 50%, preferably about 10 to about40%, by weight liquid vehicle, as noted above.

[0055] The compositions described herein may additionally comprisefurther additives known in the art, such as colorants and stainingagents, rheology modifiers, adhesion enhancers, sintering inhibitors,green-state modifiers, surfactants and the like.

[0056] In the preparation of the compositions, the particulate inorganicsolids are mixed with the liquid vehicle and dispersed with suitableequipment, such as a three-roll mill or a power-mixer, according toconventional techniques well-known in the art, to form a suspension. Theresulting composition has a viscosity generally in the range of about10-500, preferably in the range of about 10-200, more preferably in therange of about 15-100 Pa.s at a shear rate of 4 sec⁻¹, for instance, asmeasured on a Brookfield HBT viscometer using #5 spindle at 10 rpm and25° C. The general procedure for preparing the composition describedherein is set out below.

[0057] The ingredients of the paste are weighed together in a container.The components are then vigorously mixed by a mechanical mixer to form auniform blend; then the blend is passed through dispersing equipment,such as a three-roll mill, to achieve a good dispersion of particles toproduce a paste-like composition having a suitable consistency andrheology for application onto a substrate, for instance byscreen-printing. A Hegman gauge is used to determine the state ofdispersion of the particles in the paste. This instrument consists of achannel in a block of steel that is 25 μm deep (1 mil) on one end andramps up to zero depth at the other end. A blade is used to draw downpaste along the length of the channel. Scratches appear in the channelwhere the agglomerates' diameter is greater than the channel depth. Asatisfactory dispersion will give a fourth scratch point of typically10-18 μm. The point at which half of the channel is uncovered with awell-dispersed paste is between 3 and 8 μm typically. Fourth scratchmeasurements of >20 μm and “half-channel” measurements of >10 μmindicate a poorly dispersed suspension.

[0058] The compositions are then applied to a substrate usingconventional techniques known in the art, typically by the process ofscreen printing, to a wet thickness of about 20-60 μm, preferably about35-50 μm. The compositions can be printed onto the substrates either byusing an automatic printer or a hand printer in the conventional manner.Preferably, automatic screen printing techniques are employed using a200- to 325-mesh per inch screen. The printed pattern is optionallydried at below 200° C., preferably at about 150° C., for a time periodbetween about 30 seconds to about 15 minutes before firing. Firing toeffect sintering of both the inorganic binder and the finely dividedparticles of metal is preferably done in a well-ventilated belt conveyorfurnace with a temperature profile that will allow burn-off of thevehicle at about 200-500° C., followed by a period of maximumtemperature of about 500-1000° C., preferably about 600-850° C., lastingfor about 30 seconds to about 15 minutes. This is followed by a cooldowncycle, optionally a controlled cooldown cycle, to preventover-sintering, unwanted chemical reactions at intermediate temperaturesor substrate fracture which can occur from too rapid cooldown. Aluminasubstrates are particularly susceptible to fracture resulting from toorapid cooldown. The overall firing procedure will preferably extend overa period of about 2-60 minutes, with about 1-25 minutes to reach thefiring temperature, about 10 seconds to about 10 minutes at the firingtemperature and about 5 seconds to about 25 minutes in cooldown. For themanufacture of a toughened glass substrate, a controlled cooldown cycleis generally used wherein the overall firing procedure typically extendsover a period of about 2 to 5 minutes, with about 1 to 4 minutes toreach the firing temperature, followed by a rapid cooldown.

[0059] Typical thicknesses of the thick-films after firing are fromabout 3 μm to about 40 μm, preferably from about 8 μm to about 20 μm.

[0060] The compositions described herein are primarily intended for usein the manufacture of heating elements in windows such as defogging ordefrosting elements in automotive glazing, particularly backlights. Thecompositions may also be used to incorporate other conductive functionsinto the window, such as a printed aerial or antenna. However, thecoating compositions can be employed in various other applications,including printed circuits and heating elements generally. For instance,the compositions may be used as base plates in hot water heatingappliances. There is a general need within the electronics andelectrical industry for lower-cost heating elements, particularlyscreen-printable heating elements.

[0061] The following test procedures were used to evaluate thecompositions described herein.

Test Procedures Adhesion

[0062] Copper clips (obtained from Quality Product Gen. Eng. (Wickwar),UK) are soldered to the fired conductive pattern on a glass substrate(dimensions 10.2 cm×5.1 cm×3 mm) using a 70/27/3 Pb/Sn/Ag solder alloyat a soldering iron temperature of 350 to 380° C. A small quantity of amildly active rosin flux, such as ALPHA 615-25® (Alpha Metals Limited,Croydon, U.K.) may be used to enhance solder wetting and to keep thesolder and clip in place during assembly of parts, in which case theflux is applied to the solder using a shallow tray containing a thinfilm of fresh flux. Adhesion was measured on a Chattillong pull testerModel USTM at a pull speed of 0.75±0.1 inches per minute (1.91±0.25 cmper minute) and the pull strength recorded at adhesion failure. Theaverage value of adhesion failure over 8 samples was determined. Theadhesion should preferably be greater than 10 kg, more preferablygreater than 15 kg and more preferably greater than 20 kg. The principalfailure modes of adhesion are as follows:

[0063] (a) clip separates from the conductive pattern (i.e. poor solderadhesion).

[0064] (b) the conductive pattern separates from the substrate (i.e.poor substrate adhesion).

[0065] (c) glass pullout/fracture (i.e. the bonding strengths betweenthe clip and the conductive layer and between the conductive layer andthe substrate is greater than the strength of the substrate.

[0066] (d) failure within the solder.

Resistance and Resistivity

[0067] The resistance of the fired conductive pattern on a glasssubstrate (dimensions 10.2 cm×5.1 cm×3 mm) was measured using a GenRadModel 1657 RLC bridge calibrated for use between 1 and 900 Ω orequivalent. The thickness of the conductive layer is measured using athickness measuring device such as a surf-analyser (e.g. TALYSURF, whichis a contact measuring device which analyses the surface of thesubstrate in 2 dimensions using a spring loaded stylus; any change inheight will deflect the stylus and this change will then be registeredon a recorder, such as a chart recorder; the difference between the baseline and average height gives the print thickness). Resistance of thepattern is determined by placing the probe tips at the point where theconductive track meets the solder pads. The bulk resistivity(thickness-normalised) of the layer is determined by dividing themeasured resistance for the pattern by the number of squares thereinwhere the number of squares is the length of the conductive trackdivided by the width of the track. The resistivity value is obtained asmΩ/□ at a normalised thickness, herein 10 μm, and presented herein inthe units of μΩ cm.

Particle Size

[0068] Particle size in the composition is measured according to ASTMD1210-79 using a large Hegman type fineness of grind gauge.

Chemical Durability

[0069] A solution of 1% glacial acetic acid in deionised water is usedin this test. The glass substrate (50×100 mm) having thereon a firedconductive pattern is inserted into a plastic container half-filled withthe test solution. The container is then sealed and left to stand atambient temperature. The test substrates are removed after 96, 168 and336 hours, dried and then analysed by a lift test. The lift testcomprises application of a 0.75 inch (19.1 mm) wide masking tape(Niceday™) onto the substrate and then removing sharply in approximately½ second. The results of the lift test are given as the approximatepercentage of film area removed by the tape.

[0070] The invention will now be described with reference to thefollowing examples. It will be appreciated that the examples are notintended to be limiting and modification of detail can be made withoutdeparting from the scope of the invention.

EXAMPLES Examples 1-13

[0071] Conductive patterns were prepared using the method hereinbeforedescribed. The aluminium particles used were atomised aluminiumparticles (Cotronics Corporation). The silver particles were a mixtureof 50% spherical silver particles (surface area of 0.80-1.40 m²g⁻¹) and50% flake silver particles (surface area 0.60-0.90 m²g⁻¹). The glassused was Composition I in Table 1 herein. The liquid vehicle was ethylcellulose in terpineol (in a ratio of 1 to 9) combined with themonobutyl ether of diethylene glycol (sold as butyl Carbitol™). Thesubstrate was a float glass (non-tempered) substrate. The fired filmthickness was from 8 to 20 82 m. All parts were fired through a beltfurnace with a peak firing temperature of 660° C., unless otherwisespecified, with the samples spending approximately 72 s at peaktemperature. The total door-to door transit time in the furnace wasapproximately 21 minutes.

[0072] The resistivity and solder adhesion of the patterns were measuredas a function of composition in accordance with the procedures describedabove and the results are shown in Table 2 below. TABLE 2 AdhesionStrength (W) and resistivity (ρ) as a function of composition % Ag of %Glass % Al of Example solids of solids solids ρ/μΩ cm W/kg 1 87.56 4.907.54 18.50 >20 2 85.97 4.93 9.10 36.00 >20 3 84.35 4.95 10.70 90.00 >204 82.54 4.76 12.70 22.18 18.81 5 80.95 4.76 14.29 38.69 15.88 6 79.856.06 14.09 33.98 18.43 7 78.77 7.34 13.90 32.62 >20 8 77.72 8.57 13.7238.86 18.09 9 76.69 9.78 13.53 30.35 18.33 10 75.69 10.95 13.36 36.2918.78 11 72.86 14.28 12.86 43.35 17.93 12 71.08 16.38 12.55 39.89 17.2513 69.39 18.37 12.24 42.51 16.30

[0073] The data demonstrate that aluminium-containing compositions allowthe preparation of conductive patterns which exhibit increasedresistivity while maintaining solder adhesion.

What is claimed is:
 1. Use of a composition comprising finely dividedparticles of (a) an electrically-conductive material; (b) one or moreinorganic binders; and (c) aluminium, wherein components (a), (b) and(c) are dispersed in a liquid vehicle, in the manufacture of anelectrically-conductive pattern on a substrate for the purpose ofincreasing the resistivity of said electrically-conductive pattern.
 2. Amethod of increasing the resistivity of an electrically-conductivepattern which comprises utilising in the manufacture of said conductivepattern a composition comprising finely divided particles of (a) anelectrically-conductive material; (b) one or more inorganic binders; and(c) aluminium, wherein components (a), (b) and (c) are dispersed in aliquid vehicle.
 3. A use or method according to claim 1 or 2 whereinsaid liquid vehicle is an organic medium.
 4. A use or method accordingto claim 1 or 2 wherein component (c) comprises metallic aluminiumparticles.
 5. A use or method according to claim 1 or 2 whereincomponent (c) comprises particles of an aluminium-containing alloy.
 6. Ause or method according to claim 1 or 2 wherein saidelectrically-conductive particles are silver particles.
 7. A use ormethod according to claim 1 or 2 wherein substantially all particles arein the range of 0.01 to 20 μm.
 8. A use or method according to claim 1or 2 wherein the total amount of components (a), (b) and (c) is about 50to about 95% by weight of the composition.
 9. A use or method accordingto claim 1 or 2 wherein component (a) is present in amounts of about 50to about 98% by weight of the total solids present in the composition.10. A use or method according to claim 1 or 2 wherein component (b) ispresent in amounts of about 2 to about 15% by weight of the total solidspresent in the composition.
 11. A use or method according to claim 1 or2 wherein component (c) is present in amounts of about 2 to about 15% byweight of the total solids present in the composition.
 12. A use ormethod according to any preceding claim wherein said manufacture of anelectrically-conductive pattern comprises applying to a substrate acomposition comprising finely divided particles of (a) anelectrically-conductive material; (b) one or more inorganic binders; and(c) aluminium, said components (a), (b) and (c) being dispersed in aliquid vehicle, and firing the coated substrate to effect sintering ofthe finely-divided particles to the substrate.
 13. A use or methodaccording to claim 12 wherein said manufacture comprises ascreen-printing process.
 14. The use of finely-divided particles ofaluminium in a composition further comprising finely divided particlesof (a) an electrically-conductive material and (b) one or more inorganicbinders dispersed in a liquid vehicle, for the purpose of increasing theresistivity of an electrically-conductive pattern manufactured from saidcomposition.
 15. A method for increasing the resistivity of anelectrically-conductive pattern manufactured from a compositioncomprising finely-divided particles of (a) an electrically-conductivematerial and (b) one or more inorganic binders dispersed in a liquidvehicle, said method comprising the incorporation of finely-dividedparticles of (c) aluminium into said composition.